1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3 *
4 * Copyright (c) 1997 Wolfgang Helbig
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 * notice, this list of conditions and the following disclaimer in the
14 * documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28
29 #include <sys/cdefs.h>
30 __FBSDID("$FreeBSD$");
31
32 #include "calendar.h"
33
34 typedef struct date date;
35
36 static int easterodn(int y);
37
38 /* Compute Easter Sunday in Gregorian Calendar */
39 date *
easterg(int y,date * dt)40 easterg(int y, date *dt)
41 {
42 int c, i, j, k, l, n;
43
44 n = y % 19;
45 c = y / 100;
46 k = (c - 17) / 25;
47 i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
48 i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
49 j = (y + y/4 + i + 2 - c + c/4) % 7;
50 l = i - j;
51 dt->m = 3 + (l + 40) / 44;
52 dt->d = l + 28 - 31*(dt->m / 4);
53 dt->y = y;
54 return (dt);
55 }
56
57 /* Compute the Gregorian date of Easter Sunday in Julian Calendar */
58 date *
easterog(int y,date * dt)59 easterog(int y, date *dt)
60 {
61
62 return (gdate(easterodn(y), dt));
63 }
64
65 /* Compute the Julian date of Easter Sunday in Julian Calendar */
66 date *
easteroj(int y,date * dt)67 easteroj(int y, date * dt)
68 {
69
70 return (jdate(easterodn(y), dt));
71 }
72
73 /* Compute the day number of Easter Sunday in Julian Calendar */
74 static int
easterodn(int y)75 easterodn(int y)
76 {
77 /*
78 * Table for the easter limits in one metonic (19-year) cycle. 21
79 * to 31 is in March, 1 through 18 in April. Easter is the first
80 * sunday after the easter limit.
81 */
82 int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
83 24, 12, 1, 21, 9, 29, 17};
84
85 /* Offset from a weekday to next sunday */
86 int ns[] = {6, 5, 4, 3, 2, 1, 7};
87 date dt;
88 int dn;
89
90 /* Assign the easter limit of y to dt */
91 dt.d = mc[y % 19];
92
93 if (dt.d < 21)
94 dt.m = 4;
95 else
96 dt.m = 3;
97
98 dt.y = y;
99
100 /* Return the next sunday after the easter limit */
101 dn = ndaysj(&dt);
102 return (dn + ns[weekday(dn)]);
103 }
104